U.S. patent number 5,869,420 [Application Number 08/674,861] was granted by the patent office on 1999-02-09 for rewritable thermal recording medium.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Katsuyuki Naito.
United States Patent |
5,869,420 |
Naito |
February 9, 1999 |
Rewritable thermal recording medium
Abstract
A rewritable thermal recording medium comprises a recording
material containing a color former and a developer.
Recording-erasing of information is performed on the basis of a
change in the state of the recording material. The developer is
formed of a compound capable of forming a liquid crystal phase
and/or represented by general formula (I) given below ##STR1##
wherein Ar denotes a noncondensed polycyclic structure consisting
of a plurality of ring structures connected to each other via any
of a single bond, a vinylene bond and an ethynylene bond or a
condensation polycyclic structure, X denotes an ether bond, a
thioether bond, an ester bond or an amide bond, Y is an acidic
group, R is a substituted or unsubstituted alkyl group, a
substituted or unsubstituted alkenyl group, or a substituted or
unsubstituted alkynyl group, m is an integer of 1 to 3, and n is 0
or 1.
Inventors: |
Naito; Katsuyuki (Tokyo,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kawasaki, JP)
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Family
ID: |
15885717 |
Appl.
No.: |
08/674,861 |
Filed: |
July 3, 1996 |
Foreign Application Priority Data
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Jul 5, 1995 [JP] |
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7-169390 |
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Current U.S.
Class: |
503/201; 503/208;
503/225; 503/209; 503/216 |
Current CPC
Class: |
B41M
5/305 (20130101); B41M 5/3335 (20130101); B41M
5/3333 (20130101); B41M 5/3336 (20130101) |
Current International
Class: |
B41M
5/30 (20060101); B41M 5/333 (20060101); B41M
005/34 () |
Field of
Search: |
;503/201,208,209,214,216,225 |
References Cited
[Referenced By]
U.S. Patent Documents
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5663115 |
September 1997 |
Naito et al. |
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Foreign Patent Documents
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42 21 322 |
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Jan 1993 |
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DE |
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195 07 151 |
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Sep 1995 |
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DE |
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Other References
Patent Abstracts of Japan, JP 7-156540, Jun. 20, 1995..
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Primary Examiner: Hess; Bruce H.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. A rewritable thermal recording medium, comprising a recording
material containing a color former and a developer, wherein said
developer has a polycylis structure selected from the group
consisting of a noncondensed polycyclic structure formed of a
plurality of ring structures connected to each other via any of a
single bond, a vinylene bond and an ethynylene bond and a condensed
polycyclic structure, and substituent groups attached to the ring
structures at the ends of said polycyclic structure, at least one
of said substituent groups being an organic group having a
hydrocarbon chain.
2. The rewritable thermal recording medium according to claim 1,
wherein the substituent group attached to said developer has an
acidic group or a basic group.
3. The rewritable thermal recording medium according to claim 1,
wherein said developer is formed of a compound represented by
general formula (I) given below: ##STR22## wherein Ar denotes a
noncondensed polycyclic structure consisting of a plurality of ring
structures connected to each other via any of a single bond, a
vinylene bond, and an ethynylene bond or a condensation polycyclic
structure; X denotes an ether bond, a thioether bond, an ester bond
or an amide bond; Y denotes an acidic group; R denotes a
substituted or unsubstituted alkyl group, a substituted or
unsubstituted alkenyl group, or a substituted or unsubstituted
alkynyl group; m is an integer of 1 to 3; and n is 0 or 1.
4. The rewritable thermal recording medium according to claim 3,
wherein the compound represented by general formula (I) is capable
of forming a liquid crystal phase.
5. The rewritable thermal recording medium according to claim 3,
wherein R included in general formula (I) denotes a branched
hydrocarbon group.
6. The rewritable thermal recording medium according to claim 1,
wherein recording-erasing of information is performed on the basis
of at least one of the reversible transition between a crystalline
state and an amorphous state and a change in the phase separation
state.
7. A rewritable thermal recording medium, comprising at least a
color former, a developer, and a phase separation controller
serving to increase the phase separation speed, wherein said phase
separation controller is formed of a compound represented by a
general formula (III) given below:
wherein Ar denotes a noncondensed polycyclic structure consisting
of a plurality of ring structures connected to each other via any
of a single bond, a vinylene bond, an ethylene bond or a condensed
polycyclic structure; X denotes an ether bond, a thioether bond, an
ester bond or an amide bond; Z denotes a neutral polar group; R
denotes a substituted or unsubstituted alkyl group, a substituted
or unsubstituted alkenyl group, a substituted or unsubstituted
alkynyl group; m is an integer of 1 to 3; and n is 0 or 1.
8. The rewritable thermal recording medium according to claim 7,
wherein said phase separation controller is formed of a compound
capable of forming a liquid crystal phase at 25.degree. C. or
higher.
9. The rewritable thermal recording medium according to claim 7,
further comprising a matrix material.
10. The rewritable thermal recording medium according to claim 9,
wherein said phase separation controller is formed of a compound
capable of forming a liquid crystal phase at 25.degree. C. or
higher.
11. The rewritable thermal recording medium according to claim 10,
wherein said liquid crystal phase is selected from the group
consisting of a cholesteric phase, a discotic phase, a smectic
phase and a nematic phase.
12. A rewritable thermal recording medium comprising a recording
material containing a color former and a developer, wherein said
developer is formed of a compound represented by a general formula
(I) given below:
wherein Ar denotes a noncondensed polycyclic structure consisting
of a plurality of ring structures connected to each other via any
of a single bond, a vinylene bond, an ethylene bond or a condensed
polycyclic structure, X denotes an ether bond, a thioether bond, an
ester bond or an amide bond; Y denotes a hydroxyl group, a
phosphorus acid group or a sulfonic acid group; R denotes a
substituted or unsubstituted alkyl group, a substituted or
unsubstituted alkenyl group, or a substituted or unsubstituted
alkynyl group; m is an integer of 2 or 3 in the case where Y is a
hydroxyl group, and m is an integer of 1 to 3 in the case where Y
is a phosphorus acid group or a sulfonic acid group; and n is 0 or
1.
13. The rewritable thermal recording medium according to claim 12,
wherein said developer is capable of forming a liquid crystal phase
at 25.degree. C. or higher.
14. The rewritable thermal recording medium according to claim 13,
wherein said liquid crystal phase is selected from the group
consisting of a smectic phase and a nematic phase.
15. The rewritable thermal recording medium according to claim 12,
wherein R denotes a branched hydrocarbon group.
16. The rewritable thermal recording medium according to claim 12,
wherein R denotes a hydrocarbon group having 1 to 30 carbon
atoms.
17. The rewritable thermal recording medium according to claim 12,
wherein R denotes a hydrocarbon group having 4 to 22 carbon atoms.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rewritable thermal recording
medium.
2. Description of the Related Art
In recent years, with the advance of office automation, the amount
of various information has significantly increased, and the chances
of information output have also been increased with increase in the
information amount. In general, the information outputs are
classified into a hard copy output from a printer to paper sheets
and a display output. Unfortunately, in the hard copy output, a
large quantity of paper is consumed as a recording medium with
increase in the information output amount. Therefore, the hard copy
output is expected to be a problem in the future in respect of
protection of natural resources. On the other hand, the display
output requires a large scale circuit board in a display unit. This
brings about problems of portability and cost. For these reasons, a
rewritable recording medium, which is free from the above-noted
problems inherent in the conventional technique, is anticipated as
a third recording medium.
Some recording materials for such a rewritable recording medium,
which contains a color former such as a leuco dye and various acids
acting as a developer, are certainly known to the art. In the
conventional recording material of this type, a color development
and decoloring are brought about by the interaction between the
color former and the developer. For example, Japanese Patent
Disclosure No. 4-50290 discloses recording materials which contain
a leuco dye, an acid as a developer, and a long-chain amine as a
decoloring agent, and in which heat energy is supplied to the
recording material so as to repeatedly perform the chemical color
development and decoloring. Additional recording materials, which
contain a leuco dye and a long-chain phosphonic acid as a developer
and in which the heat energy is controlled so as to change the
crystal structure and, thus, to achieve reversible changes between
the color developed state and the decolored state, are disclosed
in, for example,"Pre-prints for 42nd Polymer Forum (1993, page
273)". Further,"Japan Hardcopy '93, pp 413-416" teaches an
additional type of recording material, which contains a leuco dye
and a long-chain 4-hydroxyanilide compound that is highly
crystallizable and in which reversible changes between the color
developed state and the decolored state are achieved, by supplying
heat energy, on the basis of reversible changes between the
crystalline state and amorphous state.
However, these recording materials require a very large activation
energy for repeatedly performing the color development-decoloring
functions, making it difficult to improve the recording-erasing
speed in general. Where, for example, the color development and
decoloring of the composition containing a color former and a
developer are reversibly repeated on the basis of the transition
between the crystalline state and the amorphous state, it is
difficult to improve the recording-erasing speed, because the
transition from the amorphous state, which is in a metastable
nonequilibrium state, to the crystalline state, which is in a
stable equilibrium state, takes a long time in general. On the
other hand, where the composition is prepared simply in view of
improvement in the speed of the transition from the amorphous state
to the crystalline state, it is difficult to allow the composition
to form an amorphous state of a long life. In other words, if it is
intended to improve the recording-erasing speed in the conventional
recording material containing a color former and a developer, the
color developed state or the decolored state, whichever corresponds
to the metastable nonequilibrium state, is rendered unstable. As a
result, a change from the nonequilibrium state to the equilibrium
state proceeds gradually even if a heat energy is not supplied form
the outside to the recording medium, giving rise to reduction in
the thermal stability of recording.
As described above, vigorous studies are being made on a rewritable
thermal recording medium using a recording material containing a
color former and a developer. In the conventional thermal recording
medium, however, an improvement in the recording-erasing speed is
contradictory to an improvement in the thermal stability of
recording. Since it is difficult to satisfy these requirements
simultaneously, a satisfactory rewritable thermal recording medium
has not yet been put to a practical use.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above-noted
problems inherent in the prior art so as to provide a rewritable
thermal recording medium which permits improving the
recording-erasing speed and exhibits a sufficiently high thermal
stability of recording.
According to a first aspect of the present invention, there is
provided a rewritable thermal recording medium, comprising a
recording material containing a color former and a developer,
wherein the developer has a polycyclic structure selected from the
group consisting of a noncondensed polycyclic structure formed of a
plurality of ring structures connected to each other via any of a
single bond, a vinylene bond and an ethynylene bond and a condensed
polycyclic structure, and substituent groups attached to the ring
structures at the ends of the polycyclic structure, at least one of
the substituent groups being an organic group having a hydrocarbon
chain.
It is desirable to use a compound capable of forming a liquid
crystal phase as the developer.
According to a second aspect of the present invention, there is
provided a rewritable thermal recording medium, comprising a
recording material containing at least a color former, a developer,
and a matrix agent capable of forming a liquid crystal phase.
Further, according to a third aspect of the present invention,
there is provided a rewritable thermal recording medium, in which
recording-erasing of information is performed on the basis of a
reversible change in the phase separation state of a recording
material containing at least a color former, a developer, a phase
separation controller serving to increase the phase separation
speed, and, as desired, a matrix agent, the phase separation
controller being capable of forming a liquid crystal phase.
In principle, the operation of the rewritable thermal recording
medium of the present invention is based on the interaction between
the color former and the developer contained in the recording
material. What should be noted is that the particular interaction
is controlled by controlling the reversible transition between the
crystalline and amorphous states or by controlling the change in
the phase separation state of the recording material. It is
important to note that any of the developer, matrix agent and phase
separation controller contained in the recording material should be
formed of a compound capable of forming a liquid crystal phase.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the thermal characteristics of a recording material
used in a rewritable thermal recording medium of the present
invention;
FIG. 2 shows how the state of a recording material is changed in a
rewritable thermal recording medium of the present invention;
FIG. 3 shows how the state of another recording material is changed
in a rewritable thermal recording medium of the present
invention;
FIG. 4 shows how the state of still another recording material is
changed in a rewritable thermal recording medium of the present
invention; and
FIG. 5 is a vertical cross sectional view exemplifying the
construction of a rewritable thermal recording medium of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A rewritable thermal recording medium of the present invention
comprises a recording material containing a color former, and a
developer. In a general sense, the color former denotes a precursor
compound of a coloring matter which forms a colored image, and a
developer is a compound which serves to develop a color through an
exchange of an electron or a proton with the color former. That is,
the composition system of a color former and a developer generally
develops a color when the interaction between the two increases and
loses a color when the interaction decreases. For performing the
recording-erasing of information in the rewritable thermal
recording medium of the present invention, the interaction between
the color former and the developer is controlled on the basis of
the crystallographic or thermodynamic change in state of the
recording material containing at least a color former and a
developer, i.e., on the basis of a reversible transition between
the crystalline state and the amorphous state or a change in the
state of the phase separation.
Let us describe the transition between the crystalline and
amorphous states of the recording material with reference to the
thermal characteristics shown in FIG. 1. The drawing covers a
recording material which forms a metastable amorphous state at room
temperature. If the recording material in an amorphous state is
heated to temperatures falling within a range of between the
crystallization temperature Tc and the melting point Tm, followed
by cooling the recording material, the material forms a stable
crystalline state at temperatures lower than the glass transition
temperature Tg. If the recording material in the crystalline state
is heated again to temperatures higher than the melting point Tm,
followed by subjecting the molten material to a rapid cooling or
natural cooling to room temperature lower than the glass transition
temperature Tg, the recording material is allowed to assume the
amorphous state again. It follows that, in the recording material
exhibiting the thermal characteristics as shown in FIG. 1, a
reversible transition between the crystalline state and the
amorphous state can be repeated by supplying heat energies of two
different values, which permit heating the recording material to
temperatures falling within a range of between the crystallization
temperature Tc and the melting point Tm and to temperatures higher
than the melting point Tm.
For example, where the recording material contains a color former
and a developer, these color former and developer are uniformly
mixed under the amorphous state of the recording material, with the
result that the interaction between the two is increased so as to
perform a color development. Under the crystalline state, the color
former and the developer are phase-separated from each other, with
the result that the interaction between the two is weakened so as
to perform decoloring.
In the present invention, it is possible to add a matrix agent to
the recording material containing a color former and a developer.
The term "matrix agent" used herein denotes a compound which
affects a reversible change in the state of the recording material.
To be more specific, a recording material containing a color
former, a developer, and a matrix agent is more likely to bring
about the reversible change noted above than a material containing
a color former and a developer alone. It is desirable for the
matrix agent to dissolve preferentially the developer (or the color
former). The particular property of the matrix agent permits easily
changing the interaction between the color former and the
developer, the interaction causing the color development and
decoloring in the recording material.
A recording material containing a color former, a developer and a
matrix agent is opposite in many cases in the recording-erasing
mode to a material which does not contain a matrix agent. To be
more specific, the color former and the developer are uniformly
present within the matrix agent when the recording material is in
an amorphous state, with the result that the interaction between
the color former and the developer is weakened so as to perform the
decoloring. On the other hand, the color former and the developer
are segregated at grain boundaries of the matrix agent, when the
recording material is in a crystalline state, so as to increase the
interaction between the two and, thus, to achieve a color
development. By contraries, it is possible in some cases that any
of the color former and the developer forms a mixed crystal
together with the matrix agent when the recording material is in a
crystalline state. In this case, the mixed crystal phase is
completely separated from the phase of the color former or the
developer, which is not involved in the mixed phase. As a result,
the interaction between the color former and the developer is
weakened so as to put the recording material in a decolored
state.
As described above, recording-erasing of information based on the
transition between the crystalline state and the amorphous state
can be performed by supplying heat energies of two different values
to a recording material containing a color former, a developer and,
as required, a matrix agent so as to heat the recording material to
temperatures falling within a range of between the crystallization
temperature Tc and the melting point Tm and to temperatures
exceeding the melting point Tm. In this case, the crystalline state
corresponds to a stable equilibrium state, with the amorphous state
corresponding to a metastable nonequilibrium state.
To determine whether the composition used in the present invention
is crystalline or amorphous, it is possible to employ general
methods such as an X-ray diffractometry, an electron diffractometry
and measurement of a light transmittance. When it comes to, for
example, the X-ray diffractometry or electron diffractometry, sharp
peaks or spots can be observed in the case of a crystalline
composition, though such peaks or spots cannot be observed in the
case of an amorphous composition. On the other hand, a light
scattering of the composition can be evaluated when it comes to the
measurement of a light transmittance. It should also be noted that,
where the composition is polycrystalline, the light is scattered
more strongly with decrease in the wavelength of the light, leading
to a low light transmittance. It follows that the decrease in the
light transmittance caused by the light scattering can be
distinguished from the decrease in the light transmittance caused
by the light absorption by looking into the dependence of the light
transmittance on the wavelength of light, making it possible to
estimate the grain diameter of the crystal.
In the rewritable thermal recording medium of the present
invention, it is possible for the repetition of the phase
transition between the crystalline state and the amorphous state to
take place in the entire portion or some portion of the recording
material in recording-erasing information. Also, it is possible for
every component of the recording material to form a crystalline
state individually. Alternatively, a plurality of components may
collectively form a crystalline state. The X-ray diffractometry or
electron diffractometry can also be employed for determining
whether the repetition of transition between the crystalline state
and the amorphous state takes place in the entire portion or some
portion of the recording material. Specifically, since the peak or
spot observed in the X-ray diffractometry or electron
diffractometry has a pattern inherent in the particular component
of the recording material, it is possible to specify the component
which repeats the crystalline-to-amorphous transition within the
recording material by analyzing the pattern of the peak or
spot.
FIG. 2 schematically illustrates changes in the state of a
recording material, covering the case where recording-erasing of
information is performed on the basis of the change in the state of
the phase separation in the recording material of two component
system consisting of a color former A and a developer B. The colon
":" in FIG. 2 denotes the state of interaction between the two
components of the recording material, with the asterisk "*"
denoting a fluidized state. Further, the processes in the change of
the phase separation state are denoted by straight lines.
Specifically, the solid lines denote that the two component system
is in a solid phase. On the other hand, the broken lines and the
dotted lines denote that the two component system is in a
supercooled liquid phase and an ordinary liquid phase,
respectively.
In the two component system shown in FIG. 2, the condition that the
phase of the color former A and the phase of the developer B are
separated from each other constitutes the state of equilibrium at
room temperature. Under this condition, the interaction between the
color former A and the developer B is weak. If the recording
material under this condition is heated to temperatures higher than
the melting point Tm, a large interaction is generated between the
fluidized color former A and developer B. If the molten recording
material is cooled, preferably rapidly, to room temperature, the
recording material is forcedly solidified, with the large
interaction noted above left unchanged. As a result, the recording
material is put under the condition of a metastable nonequilibrium.
Such a state of nonequilibrium exhibits a long life under
temperatures lower than the glass transition temperature Tg. It
follows that the state of nonequilibrium can be retained over a
sufficiently long period of time, if the glass transition
temperature Tg of the recording material is not lower than room
temperature.
If the recording material under the state of nonequilibrium is
heated to temperatures higher than the glass transition temperature
Tg, the diffusion speed of each of the color former A and the
developer B is rapidly increased. As a result, the phase separation
between the color former A and the developer B is promoted toward
the state of equilibrium which is most stable. In general, it
suffices to heat the recording material in this step to
temperatures falling within a range between the crystallization
temperature Tc and the melting point Tm of the recording material,
though it is desirable to determine appropriately the heating
temperature in this step in view of the diffusion speed of each of
the components of the recording material and the heat source used.
If the recording material, in which the phase of the color former A
and the phase of the developer B are sufficiently separated from
each other, is cooled again to room temperature, the recording
material is brought back to the state of equilibrium in which the
interaction between the color former A and the developer B is
weak.
FIG. 3 schematically illustrates changes in the state of a
recording material, covering the case where recording-erasing of
information is performed on the basis of the change in the state of
the phase separation in the recording material of three component
system consisting of a color former A, a developer B, and a matrix
agent C. The drawing is prepared on the assumption that the
solubility of the developer B in the matrix agent C is markedly
higher than that of the color former A in the matrix agent C under
the molten state of the recording material.
In the recording material of the three component system shown in
FIG. 3, the condition that the phase consisting of the color former
A and the developer B is separated from the phase of the matrix
agent C constitutes the state of equilibrium at room temperature.
Under this condition, a large interaction takes place between the
color former A and the developer B so as to achieve a color
development. If the recording material under the state of
equilibrium is heated to temperatures higher than the melting point
Tm, the developer B is dissolved in the matrix agent C. However,
the color former A is dissolved only slightly in the matrix agent
C. Then, if the molten recording material is cooled to room
temperature, preferably rapidly, to forcedly solidify the recording
material, the developer B is taken into the matrix agent C in a
large amount exceeding the equilibrium solubility. As a result, the
recording material assumes a metastable state of nonequilibrium in
which the interaction between the color former A and the developer
B is lost so as to achieve decoloring. The state of nonequilibrium
exhibits a long life under temperatures lower than the glass
transition temperature Tg of the recording material. It follows
that the state of nonequilibrium can be retained over a
sufficiently long period of time, if the glass transition
temperature Tg of the recording material is not lower than room
temperature.
If the recording material under the state of nonequilibrium is
heated to temperatures higher than the glass transition temperature
Tg of the recording material, the diffusion speed of the developer
B which has been taken into the matrix agent C is rapidly
increased. As a result, the phase separation between the phase of
the developer B and the phase of the matrix agent C is promoted
toward the most stable state of equilibrium of the recording
material. In general, it suffices to set the heating temperature in
this step to fall within a range of between the crystallization
temperature Tc and the melting point Tm of the recording material,
though it is desirable to determine appropriately the heating
temperature in view of the solubility of the developer B in the
matrix agent C and heat source used. If cooled again to room
temperature after a sufficient phase separation between the
developer B and the matrix agent C, the recording material tends to
be brought back to the equilibrium state. As a result, a large
interaction is generated again between the color former A and the
developer B so as to achieve a color development. It is possible in
the present invention that, when the recording material is heated
to temperatures higher than the melting point Tm so as to melt the
recording material, the solubility of the color former in the
matrix agent is higher than the solubility of the developer in the
matrix agent. It is also possible that, after cooling of the
recording material to room temperature, the color former is taken
into the matrix agent in a large amount exceeding an equilibrium
solubility.
Where the recording-erasing of information is performed on the
basis of the change in the phase separation state of the recording
material, it is effective to add a phase separation controller
serving to increase the phase separation speed of the recording
material in the rewritable thermal recording medium of the present
invention. It is desirable to use as a phase separation controller
a compound having a melting point lower than that of a two
component system consisting of a color former and a developer or a
three component system consisting of a color former, a developer,
and a matrix agent. Also, the compound used as the phase separation
controller should desirably be capable of dissolving at least one
of the color former and the developer under temperatures lower than
the melting point of the compound. Let us describe a recording
material of four component system prepared by adding a phase
separation controller to the material of the three component system
shown in FIG. 3. In this case, the speed of phase separation
between the developer and the matrix agent can be increased, as
described in the following with reference to FIG. 4.
Specifically, FIG. 4 schematically illustrates changes in the state
of a recording material containing a color former A, a developer B,
a matrix agent C, and a phase separation controller D, covering the
case where recording-erasing of information is performed on the
basis of the change in the state of the phase separation in the
particular recording material.
In the recording material of the four component system shown in
FIG. 4, the condition that the phase consisting of the color former
A and the developer B is separated from the phase of the matrix
agent C and from the phase of the phase separation controller D
constitutes the state of equilibrium at room temperature. Under
this condition, a large interaction takes place between the color
former A and the developer B so as to achieve a color development.
If the recording material under the state of equilibrium is heated
to temperatures higher than the melting point Tm, the developer B
is dissolved in the matrix agent C. However, the color former A is
dissolved only slightly in the matrix agent C. Then, if the molten
recording material is cooled to room temperature to forcedly
solidify the recording material, the developer B is taken into the
matrix agent C in a large amount exceeding the equilibrium
solubility. As a result, the recording material assumes a
metastable state of nonequilibrium in which the interaction between
the color former A and the developer B is substantially lost so as
to achieve decoloring. Particularly, the presence of the phase
separation controller D permits the recording material to be
capable of supercooling, with the result that the compatible system
consisting of the matrix agent C and the phase separation
controller D maintains a fluidity even under temperatures lower
than the melting point Tm. It follows that the recording material
is capable of assuming a metastable state of nonequilibrium even if
the recording material is cooled slowly. The state of
nonequilibrium exhibits a long life under temperatures lower than
the glass transition temperature Tg of the recording material. It
follows that the state of nonequilibrium can be retained over a
sufficiently long period of time, if the glass transition
temperature Tg of the recording material is not lower than room
temperature.
If the recording material under the state of nonequilibrium is
heated to temperatures exceeding the glass transition temperature
Tg, the diffusion speed of the developer B is rapidly increased. As
a result, the phase separation between the developer B and the
matrix agent C is promoted toward the most stable state of
equilibrium. If the recording material is further heated to
temperatures higher than the melting point TmD of the phase
separation controller D, the liquefied phase separation controller
D dissolves the developer B and a portion of the matrix agent C. As
a result, the diffusion speed of the developer B is drastically
increased so as to markedly promote the phase separation between
the developer B and the matrix agent C. If cooled again to room
temperature after the sufficient phase separation between the
developer B and the matrix agent C, the recording material is
brought back to the equilibrium state. As a result, a large
interaction is brought about again between the color former A and
the developer B so as to achieve a color development.
As described above, the presence of the phase separation controller
permits markedly increasing, for example, the speed of phase
separation between the developer B and the matrix agent C. As a
matter of fact, the speed of phase separation can be made 10.sup.2
to 10.sup.4 times as high as that of the three component system
which does not contain the phase separation controller under
temperatures in the vicinity of the glass transition temperature Tg
of the recording material. The phase separation speed can be
further increased to 10.sup.3 to 10.sup.4 times as high as that of
the particular three component system under temperatures in the
vicinity of the melting point TmD of the phase separation
controller D.
Where the phase separation controller is added to a recording
material of two component system consisting of a color former and a
developer as shown in FIG. 2, the phase separation controller also
permits increasing the speed of phase separation between the color
former and the developer.
As described above, heat energies having two different values are
supplied appropriately to the rewritable thermal recording medium
of the present invention so as to reversibly repeat the change in
the state of phase separation between two different phases of the
recording material. As a result, the degree of interaction between
the color former and the developer is changed so as to record or
erase information. The change in the state of the phase separation
noted above can be explained as a phenomenon which is generally
known to the art as a spinodal decomposition or microphase
separation.
In the present invention, a change in the state of the recording
material takes place in the form of any of the transition between
the crystalline and amorphous states and the change in the phase
separation state, when a heat energy is supplied to the
composition. Which type of the change in the state of the recording
material to take place depends not only on the kinds and
combination thereof of the color former, the developer, and the
matrix agent contained in the recording material but also on the
mixing ratio of these components. Incidentally, the type of change
in the state of the recording material can be estimated on the
basis of the change with time in the colored state of the recording
material which takes place when the recording material in a
metastable state of nonequilibrium is heated to temperatures higher
than the glass transition point Tg to cause the recording material
to be changed toward the state of equilibrium. To be more specific,
a change with time in the reflection density or light transmittance
is measured first, followed by obtaining therefrom a change with
time in the colored state of the recording material. Where the
color change follows the Arrhenius equation, a heat activation type
reversible transition between the crystalline state and the
amorphous state is considered to have taken place preferentially.
Where the color change follows the Vogel-Fulcher equation, however,
a change in the state of the phase separation is considered to have
taken place preferentially. It should be noted in this connection
that the reversible transition between the crystalline state and
the amorphous state and the change in the state of the phase
separation may take place simultaneously in some cases, though any
of the reversible transition and the change in the state of the
phase separation takes place independently in other cases in the
recording material used in the rewritable thermal recording medium
of the present invention.
In the present invention, recording-erasing of information can be
performed on the basis of the reversible transition between the
crystalline state and the amorphous state or the change in the
state of the phase separation by giving two heat histories
differing from each other in the cooling rate after the heating to
temperatures higher than the melting point Tm in place of supplying
heat energies of two different values to the recording material. To
be more specific, if the recording material heated to temperatures
higher than the melting point Tm is cooled rapidly to room
temperature, the recording material is allowed to assume a
metastable state of nonequilibrium. If cooled gradually, however,
the recording material is allowed to assume a state of equilibrium.
It follows that the transition between the crystalline state and
the amorphous state or the change in the state of the phase
separation can be repeated reversibly by suitably selecting any of
the rapid cooling or gradual cooling in the cooling step so as to
control as desired the intensity of the interaction between the
color former and the developer. Further, a stress may be applied to
the recording material, in place of supplying heat energies, in the
process of change in the state of the recording material from the
metastable state of nonequilibrium to the state of equilibrium.
To reiterate, the recording material used in the rewritable thermal
recording medium of the present invention comprises a color former,
a developer, a matrix agent, as desired, and a phase separation
controller, as desired. As already described, any of the developer,
the matrix agent and the phase separation controller should
desirably be capable of forming a liquid crystal phase.
The color former used in the present invention includes
electron-donating organic substances such as leucoauramines,
diarylphthalides, polyarylcarbinols, acylauramines, arylauramines,
Rhodamine B lactams, indolines, spiropyrans, fluorans, cyanine dyes
and Crystal Violet, and electron-accepting organic substances such
as phenolphthaleins.
To be more specific, the electron-donating organic substances
include, for example, Crystal Violet lactone (CVL), Malachite Green
lactone, Crystal Violet carbinol, Malachite Green carbinol,
N-(2,3-dichlorophenyl)leucoauramine, N-benzoylauramine, Rhodamine B
lactam, N-acetylauramine, N-phenylauramine, 2-(phenylimino
ethanediylidene)-3,3-dimethylindoline,
N-3,3-trimethylindolinobenzospiropyran,
8'-methoxy-N-3,3-trimethylindolinobenzospiropyran,
3-diethylamino-6-methyl-7-chlorofluoran,
3-diethylamino-7-methoxyfluoran, 3-diethylamino-6-benzyloxyfluoran,
1,2-benzo-6-diethylaminofluoran,
3,6-di-p-toluidino-4,5-dimethylfluoran,
phenylhydrazide-.gamma.-lactam, and 3-amino-5-methyl fluoran.
2-anilino-6-(N-cyclohexyl-N-methylamino)-3-methylfluoran,
2-anilino-3-methyl-6-(N-methyl-N-propylamino)fluoran,
3-[4-(4-phenylaminophenyl)aminophenyl]
amino-6-methyl-7-chlorofluoran,
2-anilino-6-(N-methyl-N-isobutylamino)-3-methylfluoran,
2-anilino-6-(dibutylamino)-3-methylfluoran,
3-chloro-6-(cyclohexylamino)fluoran,
2-chloro-6-(diethylamino)fluoran,
7-(N,N-dibenzylamino)-3-(N,N-diethylamino)fluoran,
3,6-bis(diethylamino)fluoran-.gamma.-(4'-nitroanilino)lactam, and
3-diethylaminobenzo [a]-fluoran.
On the other hand, the electron-accepting organic substances used
in the present invention include, for example, phenolphthalein,
tetrabromophenolphthalein, phenolphthalein ethyl ester, and
tetrabromophenolphthalein ethyl ester.
The color former compounds exemplified above can be used singly or
in the form of a mixture of a plurality of different compounds. In
the present invention, a multicolored image can be obtained because
the colored states in various colors can be attained by properly
choosing the color formers. Of the above compounds, cyanine dyes
and Crystal Violet sometimes lose a color when the interaction with
the developer is increased, and develop a color when the
interaction is decreased.
Where an electron-donating organic substance is used as the color
former, the developer used in the present invention includes acidic
compounds such as phenols, phenol metal salts, carboxylic acid,
metal carboxylates, sulfonic acids, sulfonates, phosphoric acids,
metal phosphates, acidic phosphoric esters, acidic phosphoric ester
metal salts, phosphorous acids, and metal phosphites. On the other
hand, where an electron-accepting organic substance is used as the
color former, it is desirable to use a basic compound such as
amines as the developer. Among the above developers, phenols and
phenol metal salts are especially preferable because a recording
medium containing a developer selected from them represents a high
reflection density and a high stability in a color developed state
as well as can easily repeat changes between a color developed
state and decolored state. Examples of developers used in
combination with color formers such as cyanine dyes and crystal
Violet, which serve to allow a decolored state as the interaction
between them is decreased and allow a color developed state as the
interaction is increased, are sulfonic acids, sulfonates,
phosphoric acids, metal phosphates, acidic phosphoric esters,
acidic phosphoric ester metal salts, phosphorous acids, and metal
phosphites. These compounds can be used singly or in the form of a
mixture consisting of a plurality of different compounds.
The matrix agent used in the present invention should desirably be
capable of easily forming a colorless, good amorphous state. If the
matrix agent is capable of forming a colorless and transparent
amorphous state, the contrast ratio between the printed portion and
the background can be increased. The matrix agent meeting the
particular requirement can be provided by a compound having a large
molecular weight, being small in an enthalpy change of melting
.DELTA.H of the crystal per weight and, thus, being low in its
maximum crystal growth velocity (MCV). If the crystal of the matrix
agent has a small enthalpy change of melting .DELTA.H, the heat
energy required for melting the crystal is decreased, leading to an
energy saving. Under the circumstances, it is desirable to use as
the matrix agent a compound having a bulky molecular skeleton close
to a spherical form such as the steroid skeleton.
By contraries, a low molecular compound having a molecular weight
of less than 100 is unsuitable for use as a matrix agent in the
present invention because such a low molecular compound is large in
its enthalpy change of melting .DELTA.H and unlikely to form an
amorphous state. Likewise, linear long-chain alkyl derivatives and
planar aromatic compounds are unsuitable for use as a matrix agent,
even if these compounds have a molecular weight of 100 or more. On
the other hand, a compound having a plurality of sites at which
intermolecular hydrogen bonds can be formed has a substantially
large molecular weight, even if the compound itself has a low
molecular weight or the enthalpy change of melting .DELTA.H of the
crystal of the compound is large to some extent. It follows that
the particular compound is capable of easily forming an amorphous
phase and, thus, can be used as a matrix agent in the present
invention. The substituents capable of an intermolecular hydrogen
bond formation include, for example, hydroxyl group, primary and
secondary amino groups, primary and secondary amide bonds, urethane
bond, urea bond, hydrazone bond, hydrazine group, and carboxyl
group. In other words, it is desirable to use as the matrix agent a
compound having a plurality of substituents exemplified above. On
the other hand, a compound forming an intramolecular hydrogen bond
is unsuitable for use in the present invention as a matrix agent,
even if the compound has a plurality of sites at which hydrogen
bonds can be formed.
In view of the requirements described above, it is desirable to use
sterols as a matrix agent. Specific sterols which can be used in
the present invention include, for example, cholesterol,
stigmasterol, pregnenolone, methylandrostenediol, estradiol
benzoate, epiandrostene, stenolone, .beta.-citosterol, pregnenolone
acetate, .beta.-cholestanol, 5,16-pregnadiene-3.beta.-ol-20-one,
5-.alpha.-pregnene-3.beta.-ol-20-one, 5-pregnene-3 .beta.,
17-diol-20-one 21-acetate, 5-pregnene-3 .beta., 17-diol-20-one
17-acetate, 5-pregnene-3 .beta., 17-diol-20-one 21-acetate,
5-pregnene-3 .beta., 17-diol diacetate, rockogenin, tigogenin,
esmilagenin, hecogenin, diosgenin, and derivative thereof. These
matrix agents can be used singly or a plurality of these compounds
can be used together in the form of a mixture.
Where the recording-erasing is performed in the rewritable thermal
recording medium of the present invention on the basis of a
reversible transition of the recording material between the
crystalline state and the amorphous state, it is desirable for the
matrix agent itself to be capable of repetition of a reversible
transition between the crystalline state and the amorphous state.
However, it is not absolutely necessary for the matrix agent alone
to be capable of repetition of a reversible transition between the
crystalline state and the amorphous state as far as the matrix
agent is combined with the color former or the developer so as to
reversibly repeat the particular transition. In other words, the
maximum crystal growth velocity (MCV) and the maximum crystal
growing temperature Tc, max of the recording material can be
controlled in the present invention by suitably choosing the color
former or the developer which is used in combination with the
matrix agent. Further, it is possible for a plurality of different
compounds to perform the function of the matrix agent, when mixed
together.
As already described, the matrix agent used in the present
invention should desirably be capable of preferentially dissolving
one of the color former and the developer, e.g., the developer, in
the melting step of the recording material. If the matrix agent
preferentially dissolves the developer, the interaction between the
color former and the developer can be prominently weakened when the
recording material is made amorphous by cooling so as to decrease
the color density in a decolored state, leading to an improved
contrast ratio between the printed portion and the background.
Also, if the amorphous state is colorless and highly transparent,
the printing on the underlayer can be recognized. For improving the
solubility of the developer in the matrix agent, it is desirable to
use a matrix agent which is highly compatible with the developer.
The matrix agent meeting the particular requirement can be provided
by a compound capable of forming a hydrogen bond with, for example,
the developer. Specifically, it is desirable to use compounds
having a polar group such as alcohols, thiols, carboxylic acids,
carboxylates, phosphates, sulfonates, sulfides, disulfides,
sulfoxides, sulfones, and carbonates. These compounds can be used
singly or in the form of a mixture of different compounds.
In the present invention, it is desirable to use as a phase
separation controller a low molecular organic material that is
highly crystallizable, the organic material having a hydrocarbon
group having at least 8 carbon atoms together with a polar group
such as a hydroxyl group, a carbonyl group, an ester group or a
carboxyl group. The organic materials meeting these requirements
include, for example, linear higher monohydric alcohols, linear
higher polyhydric alcohols, linear higher monovalent fatty acids,
linear higher polyvalent fatty acids, esters thereof, ethers
thereof, linear higher ketones, linear higher fatty acid amides and
linear higher polyvalent fatty acid amides.
To be more specific, the organic materials which can be used in the
present invention as the phase separation controller include, for
example, linear monohydric higher alcohols such as 1-docosanol,
1-tetracosanol, 1-hexacosanol, and 1-octacosanol; linear polyhydric
higher alcohols such as 1,12-dodecane diol, 1,14-tetradecane diol,
1,15-pentadecane diol, 1,16-hexadecane diol, 1,17-heptadecane diol,
1,18-octadecane diol, 1,19-nonadecane diol, 1,20-eicosadecane diol,
1,21-heneicosane diol, 1,22-docosane diol, 1,23-tricosane diol,
1,24-tetracosane diol, 1,12-octadecane diol, 1,2-tetradecane diol,
and 1,2-hexadecanediol; linear monovalent higher fatty acids such
as behenic acid, 1-docosanoic acid, 1-tetracosanoic acid,
1-hexacosanoic acid, and 1-octacosanoic acid; linear polyvalent
higher fatty acids such as dodecanedioic acid, and 1,12-dodecane
dicarboxylic acid; linear higher ketones such as stearone; linear
higher fatty acid alcohol amides such as isopropanolamide stearate,
isopropanolamide behenate, and hexanolamide behenate; and linear
higher fatty acid diol diesters such as ethyleglycol dilaurate,
catechol dilaurate, and cyclohexanediol dilaurate. These compounds
can be used singly or in the form of a mixture of different
compounds. A mixture which can be used as the phase separation
controller can be chosen from an ester-based wax, an alcohol-based
wax and an urethane-based wax.
In order to increase the recording-erasing speed, the phase
separation controller should desirably be capable of supercooling.
In other words, a difference between the melting point and the
solidifying point of the phase separation controller should
desirably be at least 10.degree. C. In order to improve the thermal
stability of recording in the thermal recording medium of the
present invention, the phase separation controller should desirably
have a melting point not lower than 60.degree. C.
In addition to the general materials which have already been
described in respect of the color former, the developer, the matrix
agent, and the phase separation controller, it is also possible to
use in the present invention LC-type compounds, i.e., compounds
capable of forming a liquid crystal or compounds possessing a
liquid-crystal-like molecular structure, as a developer, a matrix
agent, and a phase separation controller.
Where the recording material used in the present invention contains
an LC-type compound, the change from the metastable state of
nonequilibrium to the state of equilibrium can be brought about in
a short time. Suppose the recording-erasing of information is
performed on the basis of transition of the recording material from
the crystalline state to the amorphous state. If the recording
material containing an LC-type compound is heated from the
amorphous state, which is in the metastable state of
nonequilibrium, to temperatures exceeding the crystallization
temperature Tc, the recording material readily forms a state having
a high degree of order such as a liquid crystal phase. It follows
that it is possible to decrease the potential energy in
crystallizing the recording material under the above state, making
it possible to achieve the crystallization in a short time. A
similar effect can also be obtained where the recording-erasing of
information is performed on the basis of the change in the phase
separation state of the recording material. Specifically, if the
recording material is heated to temperatures exceeding the
crystallization temperature Tc, the LC-type compound forms a state
having a high degree of order. At the same time, the diffusion
speed of the color former or the developer is increased. As a
result, the phase separation speed of the recording material is
increased. It follows that the potential energy between the two
phase separation states is decreased so as to permit the change
between these two states to be achieved in a short time. In
addition, since the LC-type compound exhibits a large
intermolecular attractive force and a high thermal stability, the
recording material containing an LC-type compound exhibits a
sufficiently high thermal stability even under an amorphous
state.
In order to increase the recording-erasing speed, the LC-type
compound used as a developer, a matrix agent or a phase separation
controller should desirably be capable of forming a liquid crystal
phase in the form of a supercooled liquid under temperatures lower
than the melting point, because the transition between the liquid
crystal phase and the crystalline state is performed in a very
short time. In view of the thermal stability of the recording
material under the state of nonequilibrium, the LC-type compound
should be capable of forming a liquid crystal phase at temperatures
not lower than 25.degree. C., preferably not lower than 60.degree.
C., though the crystallization temperature of the LC-type compound
is not particularly limited where the recording medium is used for
a special purpose at room temperature or lower.
It is not absolutely necessary for the LC-type compound used in the
present invention to form a liquid crystal phase. In other words,
the recording-erasing speed can be increased, as far as the LC-type
compound permits forming a state having a high degree of order to
some extent. It should be noted, however, that, where the LC-type
compound forms a liquid crystal phase at high temperatures
exceeding 200.degree. C., a large energy is required for heating
the recording material to permit the LC-type compound to be
fluidized sufficiently in performing the recording-erasing of
information, resulting in failure to achieve a sufficient energy
saving. Incidentally, the recording-erasing speed can be further
improved, if an electric field or a magnetic field is applied to
the recording material while supplying a heat energy to the
recording material when the recording material is heated for
forming a liquid crystal layer.
Let us describe the LC-type compounds, i.e., compounds capable of
forming liquid crystal phases, suitable for use in the present
invention as a developer, a matrix agent or a phase separation
controller.
Specifically, the LC-type compound used as a developer in the
present invention should have a polycyclic structure including
selected from the group consisting of a noncondensed polycyclic
structure formed of a plurality of ring structures connected to
each other via any of a single bond, a vinylene bond and an
ethynylene bond and a condensed polycyclic structure, and
substituent groups attached to the ring structure at the ends of
the polycyclic structure, at least one of the substituent groups
being an organic group having a hydrocarbon chain. When used in
combination with ban electron-donating color former, the developer
formed of an LC-type compound should have an acidic substituent
group. On the other hand, when used in combination with an
electron-accepting color former, the developer formed of an LC-type
compound should have a basic substituent group.
The noncondensed polycyclic structure consisting of a plurality of
ring structures connected to each other via any of a single bond, a
vinylene bond and a ethynylene bond or condensed polycyclic
structure is known to the art as a mesogen group of a liquid
crystal molecule. In other words, an LC-type compound adapted for
use as a developer can be obtained by selecting appropriately the
substituent groups attached to the ends of the mesogen group. It is
possible for a halogen atom or an alkyl group to be substituted for
a hydrogen atom in the vinylene bond. It is particularly desirable
to use an LC-type compound having a polycyclic structure consisting
of a plurality of ring structures which are connected to each other
via an ethynylene bond, because the particular LC-type compound
exhibits a high thermal stability and forms a supercooled liquid
phase of a low viscosity. On the other hand, it is undesirable for
a plurality of ring structures to be connected to each other via
linkage groups other than a single bond, a vinylene bond and an
ethynylene bond. In the case of using a linkage group which is
likely to bring about a conformation change such as an alkylene
bond, it is impossible to obtain a rigid mesogen group. Also, in
the case of using a linkage group which is likely to be hydrolyzed
such as an azo group, an azomethine group or an ester bond, the
thermal stability is rendered insufficient.
In the LC-type compound described above, a large enthalpy change
.DELTA.H is provided between the metastable state of nonequilibrium
and the state of equilibrium, because a large interaction is
performed between the mesogen groups of the adjacent molecules. In
addition, since substituent groups are attached to both ends of the
mesogen group, the LC-type compound readily forms stably a state
having a high degree of order. Further, since the mesogen group is
very rigid and is small in its change of conformation, the enthalpy
change is small when the degree of order of the molecules is
enhanced. It follows that a particularly large free energy is not
required in the process of change of the state from the state of
nonequilibrium to the state of equilibrium. In other words, the
potential barrier between the amorphous state and the crystalline
state or between the two different states of phase separation can
be lowered in the case of using a developer having the particular
molecular structure described above, making it possible to bring
about in a short time a change of state between the amorphous state
and the crystalline state or between the two different states of
phase separation.
The LC-type compound used as a developer in the present invention
is represented by general formula (I) given below: ##STR2## where
Ar is a noncondensed polycyclic structure consisting of a plurality
of ring structures connected to each other via any of a single
bond, a vinylene bond and ethynylene bond or a condensed polycyclic
structure; X is an ether bond, a thioether bond, an ester bond or
an amide bond; Y is an acidic substituent group; R is a substituted
or unsubstituted alkyl group, a substituted or unsubstituted
alkenyl group or a substituted or unsubstituted alkynyl group; m is
an integer of 1 to 3, and n is 0 or 1.
In the above formula, Y and R are substituent groups which are
attached to the ring structures at the ends of the polycyclic
structure.
The ring structure included in the compound represented by general
formula (I) may be either an aromatic ring or a saturated ring.
Also, the ring structure may contain a hetero atom. Where the ring
structure forms a hetero ring, a large intermolecular interaction
takes place, with the result that a state having a high degree of
order can be formed easily when the compound is heated. To be more
specific, Ar in general formula (I) denotes a noncondensed
polycyclic structure or a condensation polycyclic structure. The
noncondensed polycyclic structure consists of aromatic rings such
as benzene rings or saturated rings having structural formulas
exemplified below, said aromatic rings or saturated rings being
connected to each other via a single bond, a vinylene bond or an
ethynylene bond: ##STR3##
On the other hand, the condensed polycyclic structure includes, for
example, a naphthalene ring, an azulene ring, an indene ring, a
biphenylene ring, an anthracene ring, a fluorene ring, and a
phenanthrene ring.
The LC-type compound having a saturated ring is generally
advantageous in forming a liquid crystal phase having a low
viscosity. In the case of using such an LC-type compound as a
developer, the diffusion speed of the color former is increased so
as to increase the phase separation speed between the developer and
the color former. Particularly preferred is an LC-type compound
having a cyclohexane ring because a liquid crystal phase having a
low viscosity can be formed. LC-type compounds having other
saturated rings are also preferred because these LC-type compounds
permit imparting a high heat resistance to the recording
material.
The LC-type compounds represented by general formula (I), which are
used as a developer in the present invention, has a highly
crystalline noncondensed polycyclic structure or condensed
polycyclic structure, which is included in the molecular skeleton,
and exhibits a high melting point. Naturally, these LC-type
compounds permit improving the thermal stability of recording. The
noncondensation polycyclic structure is preferred because the
compound having a noncondensed polycyclic structure can easily form
a liquid crystal phase. Also, it is desirable for the
noncondensation polycyclic structure to have at least one aromatic
ring, because the resultant compound is enabled to exhibit a high
melting point. It is particularly desirable for the noncondensed
polycyclic structure to have at least one p-phenylene ring which is
rich in linearity and small in polarization, because the resultant
compound is enabled to form a liquid crystal phase quite easily. To
be more specific, the polycyclic structures effective in the
present invention include, for example, a polycyclic structure
consisting of a plurality of p-phenylene rings which are connected
to each other via a single bond, a vinylene bond or an ethynylene
bond, and a polycyclic structure consisting of at least one
p-phenylene ring and any of the saturated rings shown in the above
formula, which are connected to each other via a single bond, a
vinylene bond or an ethynylene bond. Incidentally, a condensed
polycyclic structure forming a rigid mesogen group is effective for
improving the heat resistance of the recording material.
It is particularly desirable for X included in general formula (I)
to represent an ether bond or a thioether bond, which is thermally
stable and is unlikely to be decomposed by, for example, water.
Further, the compound in which X represents an ester bond or an
amide bond is substantially free from problems depending on the use
of the resultant recording medium and is advantageous in that the
LC-type compound can be synthesized easily and at a low cost. Y in
general formula (I) represents an acidic substituent group. Since
phenols are preferred for use as a developer in the present
invention as already described, Y should desirably be a hydroxyl
group. Further, phenol group as Y may be connected to the
polycyclic structure via a linkage group such as an ester group.
The additional acidic groups represented by Y include, for example,
a phosphoric group and a sulfonic group.
R in general formula (I) represents a hydrocarbon chain having 1 to
30 carbon atoms, preferably 4 to 22 carbon atoms. If the
hydrocarbon chain is unduly long, the enthalpy change is increased
when the degree of order of the molecules is enhanced, with the
result that a large activation free energy is required for the
change of state from the state of nonequilibrium to the state of
equilibrium. Incidentally, linear alkyl derivatives tend have a
lower melting point with decrease in the length of the hydrocarbon
chain. It follows that, if a linear alkyl derivative having a short
hydrocarbon chain, which has a low melting point, is added to the
composition forming the recording material, the thermal stability
of the resultant recording medium is lowered. However, the compound
represented by general formula (I) exhibits a sufficiently high
melting point even if R has 10 or less carbon atoms, because the
compound has a noncondensed polycyclic structure or a condensed
polycyclic structure, which is included in the molecular skeleton.
It follows that the thermal recording material containing the
above-noted compound exhibits a sufficiently high thermal stability
of recording. Where R represents a perfluoroalkyl group or a
perfluoroalkylene group, the rigidity of the molecule is improved,
with the result that the compound, when heated, is enabled to form
a state having a high degree of order easily and stably. It is also
desirable to use compounds of general formula (I), in which R
represents a branched hydrocarbon group. Where, for example, a
compound, in which R represents a branched alkyl group having two
branches each having a long chain, is used as a developer, these
long branches are twisted about each other so as to improve the
thermal stability under the amorphous state. In addition, the
crystallization speed is increased.
The molecular arrangement in the liquid crystal phase formed by the
LC-type compound, i.e., compound capable of forming a liquid
crystal phase, is not particularly limited in the present
invention. If a smectic phase is formed in the case of using an
LC-type compound as a developer, a phase separation of the
recording material is facilitated because of a high degree of
order, leading to a high contrast ratio between the color-developed
state and the decolored state. On the other hand, where a nematic
phase is formed, the diffusion speed of the color former within the
developer is increased because of the low viscosity, leading to a
high phase separation speed.
The LC-type compounds suitable for use as a matrix agent include,
for example, steroid derivatives forming a cholesteric phase,
compounds having a plurality of alkyl groups attached to a rigid
aromatic ring and forming a discotic phase, and compounds forming a
smectic phase or a nematic phase and having both a rigid mesogen
group and a flexible hydrocarbon chain. Where the liquid crystal
phase formed by the particular matrix agent assumes a smectic phase
or discotic phase each having a high degree of order, the phase
separation of the recording material is facilitated so as to
improve the contrast ratio between the color-developed state and
the decolored state. Also, where the particular liquid crystal
phase assumes a nematic phase or a cholesteric phase each having a
low viscosity, the diffusion speed of the color former within the
developer is increased so as to markedly increase the phase
separation speed between the color former and the developer.
The steroid derivatives effective for forming the cholesteric phase
include, for example, a halide of cholesterol, monocarboxylic acid
cholesterol ester, monocarboxylic acid sitosterol ester, and
cholesterol ester of benzoic acid derivatives. To be more specific,
these sterol derivatives include, for example, cholesteryl
chloride, cholesteryl acetate, cholesteryl nonanate, methyl
cholesteryl carbonate, ethyl cholesteryl carbonate, cholesteryl
p-methoxy benzoate, sitosteroyl benzoate, sitosteroyl p-methyl
benzoate, and cholestanyl benzoate.
The compounds forming a discotic phase, which have a plurality of
alkyl groups attached to a rigid aromatic ring, are represented by
general formulas (IIA) and (IIB) given below: ##STR4## where R
represents n--C.sub.m H.sub.2m+1 --COO--, in which m is an integer
of 3 to 20; ##STR5## where R represents n--C.sub.m H.sub.2m+1
--COO--, n--C.sub.m H.sub.2m+1 --O--, or ##STR6## in which m is an
integer of 3 to 20.
The LC-type compounds adapted for use in the present invention as a
phase separation controller are represented by general formula
(III) given below: ##STR7## where Ar is a noncondensed polycyclic
structure consisting of a plurality of ring structures which are
connected to each other via any of a single bond, a vinylene bond
or an ethynylene bond or condensation polycyclic structure; X is an
ether bond, a thioether bond, an ester bond or an amide bond; Z is
a neutral polar group; R is a substituted or unsubstituted alkyl
group, a substituted or unsubstituted alkenyl group, a substituted
or unsubstituted alkynyl group, or a substituted or unsubstituted
aryl group; m is an integer of 1 to 3; and n is 0 or 1.
In the rewritable thermal recording medium of the present
invention, it is important to pay attentions to the glass
transition temperature Tg of the recording material. If the glass
transition temperature Tg is low, i.e., about room temperature, the
phase separation or crystallization of the recording material,
which is caused by the diffusion of the color former or the
developer, is likely to be brought about by a slight elevation of
the ambient temperature, with the result that the thermal stability
of recording tends to be lowered. It follows that it is necessary
for the recording material, in which an amorphous phase is formed
in the entire region or in a portion, to have a glass transition
temperature Tg of at least 25.degree. C., preferably at least
50.degree. C. In view of this requirement, it is desirable to use
as a color former a compound having a bulky molecular skeleton
close to, for example, a spherical form, having a large molecular
weight and being small in its enthalpy change of melting .DELTA.H.
A compound having a plurality of sites at which intermolecular
hydrogen bonds can be formed is also adapted for use as a color
former in the present invention.
By contraries, in the case of using a recording material having a
glass transition temperature Tg close to, for example, room
temperature, it is possible to provide a rewritable thermal
recording medium which permits the recorded information to be
naturally erased after storage in the recording medium for a
predetermined period of time. Further, where the recording medium
is used for a special purpose, it is possible to use a recording
material having a glass transition temperature Tg lower than room
temperature. As such a special purpose, it may be possible to
display the change in the color-developed state in the case where a
temporary temperature elevation takes place in a refrigerator
during transportation of the refrigerator which houses materials
requiring refrigeration.
On the other hand, where the recording material has an unduly high
glass transition temperature Tg, a large energy is required for
heating the recording material to temperatures falling within a
range of between the crystallization temperature Tc and the melting
point Tm or to temperatures higher than the melting point Tm in the
step of recording-erasing information, resulting in failure to
achieve an energy saving. It follows that it is desirable for the
recording material used in the present invention to have a glass
transition temperature Tg not higher than 150.degree. C.
It is generally known to the art that a mixture exhibits a glass
transition temperature close to a weight average value of the glass
transition temperatures of the components of the mixture.
Therefore, in order to set the glass transition temperature Tg of
the recording material used in the present invention at a desired
value, it is desirable for the compound used as each of the color
former, the developer, and the matrix agent to exhibit a glass
transition temperature not lower than 25.degree. C., preferably not
lower than 50.degree. C. Further, in view of the thermal stability
of recording, it is desirable to use a matrix agent having a
melting point of at least 100.degree. C.
It is possible to use, for example, a differential scanning
calorimeter (DSC) for measuring the glass transition temperature Tg
in respect of the entire recording material and each of the
components of the recording material. It is also possible to
measure the glass transition temperature Tg for a portion of the
recording material by using a DSC.
On the other hand, where a component compound of the recording
material is likely to form an amorphous phase having a clear glass
transition temperature Tg, the relationship Tg=a.Tm (where a is
0.65 to 0.8) is generally known to reside between the glass
transition temperature Tg (absolute temperature) and the melting
point Tm (absolute temperature) of the component compound. It
follows that, if the glass transition temperature Tg of each of the
components of the recording material is set at a high value, the
melting point Tm of the recording material is rendered high. In
this case, the thermal stability of recording can be improved.
However, it is necessary to heat the recording material to high
temperatures for melting the recording material. It follows that it
is necessary to use a substrate having a high heat resistance,
making it difficult to put the recording medium to practical use.
In order to overcome this difficulty, it is effective to use a
recording material capable of forming a plurality of crystal forms.
For preparing a recording material capable of forming a plurality
of crystal shapes, it is desirable to use a developer or a matrix
agent capable of forming a plurality of crystal forms.
Let us describe preferred mixing ratio of the color former, the
developer, the matrix agent, and the phase separation controller in
the rewritable thermal recording medium of the present
invention.
Where the recording material of the present invention contains a
color former and a developer, but does not contain a matrix agent,
the developer should be used in an amount of 0.1 to 100 parts by
weight, preferably 1 to 10 parts by weight, relative to 1 part by
weight of the color former. If the amount of the developer is
smaller than 0.1 part by weight, it is difficult to increase
sufficiently the interaction between the color former and the
developer in the recording or erasing step. On the other hand, if
the mixing amount of the developer is larger than 100 parts by
weight, the color density tends to be lowered under the
color-developed state.
Where the recording material contains a color former, a developer
and a matrix agent, the developer should be used in an amount of
0.1 to 10 parts by weight, preferably 1 to 2 parts by weight,
relative to 1 part by weight of the color former. If the amount of
the developer is smaller than 0.1 part by weight, it is difficult
to increase sufficiently the interaction between the color former
and the developer in the recording or erasing step. On the other
hand, if the mixing amount of the developer exceeds 10 parts by
weight, it is difficult to decrease sufficiently the interaction
between the developer and the color former in the recording or
erasing time.
It is desirable to use the matrix agent in an amount of 1 to 200
parts by weight, preferably 10 to 100 parts by weight, relative to
1 part by weight of the color former. If the amount of the matrix
agent is smaller than 1 part by weight, it is difficult to cause
the transition between the crystalline state and the amorphous
state or the change in the state of the phase separation. If the
amount of the matrix agent exceeds 200 parts by weight, however,
the color density in the color developing step is lowered.
Further, it is desirable to use the phase separation controller in
an amount of 0.1 to 100 parts by weight, preferably 1 to 50 parts
by weight, relative to 1 part by weight of the color former. If the
amount of the phase separation controller is smaller than 0.1 part
by weight, a satisfactory improvement cannot be obtained in the
phase separation speed of the recording material. If the amount
exceeds 100 parts by weight, however, the state of nonequilibrium
of the recording material is rendered unstable, with the result
that the thermal stability of recording tends to be lowered.
In the rewritable thermal recording medium of the present
invention, it is possible to add, as required, a pigment, a
fluorescent dye, an ultraviolet absorber, a heat insulating agent,
a heat accumulating agent, etc. to the recording material
consisting of the color former, the developer, the matrix agent and
the phase separation controller. If, for example, a pigment is
selected appropriately in view of the color former contained in the
recording material, it is possible to obtain a desired colored
state in each of the color-developed state and the decolored
state.
In order to use the rewritable thermal recording medium of the
present invention in the form of a bulk, a recording material
containing the particular components described previously is melted
in a solventless condition for the mixing purpose, followed by
solidifying the recording material by a rapid cooling or a natural
cooling. A recording medium of a desired shape can be obtained by
shaping the molten recording material by using a mold. It is also
possible to obtain a recording medium in the form of a thin film by
expanding the molten recording material to form a thin layer. A
recording medium in the form of a thin film can also be obtained by
dissolving the recording material in a suitable solvent, followed
by casting the resultant solution. It is desirable for the thin
film thus formed to have a thickness of 0.5 to 50 .mu.m. If the
film is unduly thin, the resultant rewritable thermal recording
medium tends to fail to develop color in a sufficiently high
density. If the film is unduly thick, however, a large heat energy
is required in the recording-erasing step, making it difficult to
perform the recording-erasing operation at a high speed.
In order to improve the mechanical strength of the rewritable
thermal recording medium of the present invention, it is possible
to have the recording material used in the present invention
supported by a suitable medium. For example, the recording material
may be impregnated in a polymer sheet, may be dispersed in a binder
polymer, may be dispersed in an inorganic glass, may be impregnated
in a porous substrate, may be intercalated in a layered material,
or may be encapsulated.
In order to allow a polymer sheet to be impregnated with the
recording material of the present invention, a polymer sheet having
inner spaces large enough to hold the recording material is
impregnated with the recording material melted in the absence of a
solvent or a solution prepared by dissolving the recording material
in a suitable solvent. In view of the uniformity of the surface of
the resultant rewritable thermal recording medium, it is desirable
to use a polymer having a high wettability with the molten
recording material or the solution. The specific polymers used in
the present invention include, for example, polyether-ether
ketones; polycarbonates; polyallylates; polysulfones; ethylene
tetrafluoride resins; ethylene tetrafluoride copolymers such as an
ethylene tetrafluoride-perfluoro alkoxyethylene copolymer, an
ethylene tetrafluoride-perfluoroalkyl vinyl ether copolymer,
ethylene tetrafluoride-propylene hexafluoride copolymer, and an
ethylene tetrafluoride-ethylene copolymer; ethylene chloride
trifluoride resins; vinylidene fluoride resins; fluorine-containing
polybenzoxazoles; polypropylenes; polyvinyl alcohols;
polyvinylidene chlorides; polyesters such as polyethylene
terephthalate, polybutylene terephthalate, and polyethylene
naphthalate; polystyrenes; polyamides such as Nylon 66; polyimides;
polyimidoamides; polyether sulfones; polymethylpentenes; polyether
imides; polyurethanes; polybudatienes; celluloses such as methyl
cellulose, ethyl cellulose, cellulose acetate and nitrocellulose;
gelatins; gum arabic; and papers such as neutral paper and acidic
paper. It is particularly desirable to use celluloses and neutral
paper because these media can be easily impregnated with the molten
recording material or the solution of the recording material of the
present invention. In addition, the resultant rewritable thermal
recording medium is enabled to exhibit a high color density under
the color developed state and a low residual color density under
the decolored state.
For dispersing the recording material of the present invention in a
binder polymer, a molten recording material or a solution of the
recording material of the present invention is dispersed together
with the binder polymer and additional components, as required, by
various dispersion methods. The resultant dispersion may be coated
on a suitable substrate.
The binder polymers used in the present invention include, for
example, polyethylenes; chlorinated polyethylenes; ethylene
copolymers such as ethylene-vinyl acetate copolymer,
ethylene-acrylic acid-maleic anhydride copolymer; polybutadienes;
polyesters such as polyethylene terephthalate, polybutylene
terephthalate, and polyethylene naphthalate; polypropylenes;
polyisobutylenes; polyvinyl chlorides; polyvinylidene chlorides;
polyvinyl alcohols; polyvinyl acetals; polyvinyl butylals;
tetrafluoroethylene resins; trifluorochloroethylene resins;
ethylene fluoride-propylene resins; vinylidene fluoride resins;
vinyl fluoride resins; tetrafluoroethylene copolymers such as
tetrafluoroethylene-perfluoroalkoxyethylene copolymer,
tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,
tetrafluoroethylene-hexafluoro propylene copolymer and
tetrafluoroethylene-ethylene copolymer; fluoro resins such as
fluorine-containing polybenzoxazol; acrylic resins; methacrylic
resins such as polymethyl methacrylate; polyacrylonitriles;
acrylonitrile copolymers such as acrylonitrile-butadiene-styrene
copolymer; polystyrenes; halogenated polystyrenes; styrene
copolymers such as styrene-methacrylic acid copolymer and
styrene-acrylonitrile copolymer; acetal resins; polyamides such as
Nylon 66; gelatin; gum arabic; polycarbonates; polyester
carbonates; cullulose-based resins; phenol resins; urea resins;
epoxy resins; unsaturated polyester resins; alkyd resins; melamine
resins; polyurethanes; diallyl phthalate resins;
polyphenyleneoxides; polyphenylenesulfides; polysulfones;
polyphenylsulfones; silicone resins; polyimides; bismaleimide
triazine resins; polyimidoamide resins; polyether sulfones;
polymethyl pentenes; polyether ether ketones; polyether imides;
polyvinyl carbazols; and thermoplastic resins such as
norbornene-based amorphous polyolefins.
The dispersion methods used in the present invention include, for
example, a mixer method, a sand mill method, a ball mill method, an
impeller mill method, a colloid mill method, a three roll mill
method, a kneader method, a two roll method, a Banbury mixer
method, a homogenizer method and a nanomizer method. These
dispersion methods can be selected appropriately in view of the
viscosity of the molten recording material or solution of the
recording material, as well as the use and type of the rewritable
thermal recording medium. Further, the coating methods for coating
a substrate with the recording material of the present invention
include, for example, a spin coating method, a draw-up coating
method, an air doctor coating method, a blade coating method, a rod
coating method, a knife coating method, a squeeze coating method,
an impregnation coating method, a reverse roll coating method, a
transfer coating method, a gravure coating method, a kiss roll
coating method, a cast coating method, a spray coating method, a
curtain coating method, a calender coating method, an extrusion
coating method and an electrostatic coating method. These coating
methods can also be selected appropriately in view of the use and
type of the rewritable thermal recording medium aimed at.
Where the recording material forming a recording medium of the
present invention is dispersed in a binder polymer, the binder
polymer should be used in an amount of 0.01 to 100 parts by weight,
preferably 0.05 to 20 parts by weight, relative to 1 part by weight
of the matrix agent. If the amount of the binder polymer is smaller
than 0.01 part by weight, it is impossible to improve sufficiently
the mechanical strength of the resultant recording medium. If the
amount of the binder polymer exceeds 100 parts by weight, however,
the color density in the color developed state of the recording
medium tends to be lowered.
Where the recording material forming the recording medium of the
present invention is allowed to be supported by an inorganic glass,
it is desirable to use an inorganic glass manufactured by a
so-called sol-gel method. In this case, it is desirable for the
gelling temperature not to be unduly high. Further, the porous
substrates which can be used in the present invention include, for
example, various inorganic compounds. On the other hand, the
stratifying materials which can be used in the present invention
include, for example, mica, clay mineral, talc and chlorite.
For preparing microcapsules having the recording material of the
present invention wrapped therein, it is possible to employ an
interfacial polymerization method, an in-situ polymerization
method, an in-liquid hardening covering method, a phase separation
method from an aqueous solution system, a phase separation from an
organic solution system, an in-gas suspension method, and a spray
drying method. These methods can be properly chosen depending on
the use and type of the rewritable thermal recording medium aimed
at. The materials used in the present invention for forming the
shell of the microcapsule include, for example, condensation
polymers such as melamine resins, epoxy resins, urea resins, phenol
resins, and furan resins; thermosetting resins such as
styrene-divinyl benzene copolymer and methyl acrylate-vinyl
acrylate copolymer, which are crosslinked in three dimensional
directions; and thermoplastic resins which have already been
exemplified as binder polymers in which the recording material of
the present invention is dispersed. It is possible to form a shell
of multi-layer structure by using a plurality of different resins
selected from the thermosetting resins and the thermoplastic resins
exemplified above. In this case, it is desirable to use a
thermosetting resin for forming the outermost layer of the shell of
the microcapsule in order to improve the thermal stability of the
microcapsule. It is also possible to disperse the resultant
microcapsules in the binder polymer or the inorganic glass
exemplified above. It should be noted that, even if the recording
material itself is unlikely to be dispersed sufficiently in the
supporting medium such as the inorganic glass, a satisfactory
dispersion can be obtained in the case of dispersing the
microcapsules in the supporting medium.
How to use the rewritable thermal recording medium of the present
invention is not particularly limited. For example, the recording
medium can be used as a bulk, in combination with a supporting
medium such as fibers, or in the form of a thin film formed on a
suitable substrate. Of course, the thin film noted above acts as a
recording layer. The substrate on which a thin film of the
recording material is formed in the present invention includes, for
example, plastic films such as a polyethylene terephthalate film, a
plastic plate, a metal plate, a semiconductor substrate, a glass
plate, a wooden plate, a paper sheet, and an OHP sheet. It is also
possible to coat the substrate with the microcapsules described
previously, which are converted into a paint or an ink, followed by
drying the paint or the ink, as required. In this case, different
kinds of color formers can be wrapped in different microcapsules so
as to achieve a desired color development easily. It is also
possible to mix at a desired mixing ratio microcapsules containing
different types of color formers, having different crystallization
temperature Tc or different melting points Tm, and differing from
each other in the state exhibited by the nonequilibrium state,
i.e., whether the nonequilibrium state exhibits a color-developed
state or decolored state. In this case, the colored state can be
controlled in accordance with the magnitude of a supplied thermal
energy. It follows that a full-color recording using color formers
of, e.g., cyan, magenta and yellow can be achieved.
In the rewritable thermal recording medium of the present
invention, it is also possible to form a protective layer on the
recording layer made of a thin film of the recording material
specified in the present invention for improving the durability of
the recording layer or preventing the recording layer from being
stuck to a thermal printer head (TPH) used for supplying a heat
energy to the recording layer. The materials of the protective
layer include, for example, a wax, a thermoplastic resin, a
thermosetting resin, a photocurable resin, a water-soluble resin,
and a latex. The thickness of the protective layer should desirably
be 0.1 to 100 .mu.m. Further, the protective layer may be allowed
to contain additives such as a mold release agent, a lubricant, a
heat-resistant material, and an antistatic agent. To be more
specific, the recording layer may be coated with a dispersion or
solution containing these additives together with the recording
material specified in the present invention, followed by drying the
coating to form the particular protective layer. Alternatively, a
heat resistant film having an adhesive coated thereon in advance
may be bonded to the recording layer by a dry laminate method to
form the protective layer in question. Further, it is desirable to
form an undercoat layer between the substrate and the recording
layer in order to improve the bonding strength between the
substrate and the recording layer and to improve the solvent
resistance of the recording medium.
The heat resistant films used in the present invention are not
particularly limited as far as the film has a thermal deformation
temperature higher than the melting point of the recording material
used as a recording material. For example, high molecular compounds
can be used for forming these heat resistant films, including
polyether-ether ketones; polycarbonates; polyallylates;
polysulfones; tetrafluoroethylene resins; tetrafluoroethylene
copolymers such as tetrafluoroethylene-perfluoro alkoxyethylene
copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and
tetrafluoro ethylene-ethylene copolymer; trifluorochloroethylene
resins; vinylidene fluoride resins; fluorine-containing
polybenzoxazoles; polypropylenes; polyvinyl alcohols;
polyvinylidene chlorides; polyesters such as polyethylene
terephthalate, polybutylene terephthalate, and polyethylene
naphthalate; polystyrenes; polyamides such as Nylon 66; polyimides;
polyimidoamides; polyethersulfones; polymethylpentenes; polyether
imides; polyurethanes; and polybutadienes. These high molecular
materials can be selected appropriately in view of the use and type
of the heat source and the resultant rewritable thermal recording
medium. Further, the adhesives generally used in the dry laminate
method can be used in the present invention including, for example,
acrylic resins; phenoxy resins; ionomer resins; ethylene copolymers
such as ethylene-vinyl acetate copolymer, and ethylene-acrylic
acid-maleic anhydride copolymer; polyvinyl ethers; polyvinyl
formals; polyvinyl butyrals; gelatin; gum arabic; polyesters;
polystyrenes; styrene copolymers such as styrene-acrylic acid
copolymer; vinyl acetate resins; polyurethanes; xylene resins;
epoxy resins; phenolic resins; and urea resins.
In order to perform the recording-erasing in the rewritable thermal
recording medium of the present invention on the basis of
transition between the crystalline state and the amorphous state or
the change in the state of phase separation, heat energies having
two different values are supplied to the recording medium, as
already described. Alternatively, two kinds of heat histories
differing from each other in the cooling rate after the heating of
the recording medium to temperatures higher than the melting point
Tm are applied to the recording medium, as already described.
It is desirable to use a TPH (thermal printer head) or a laser beam
as a heat source for supplying heat energies to the recording
medium in the recording step. The TPH, which is not amazingly high
in resolution, permits heating the rewritable thermal recording
medium over a large area, and is advantageous in miniaturizing the
apparatus. On the other hand, a laser beam easily permits a high
density recording by diminishing the beam spot diameter, and also
permits increasing the recording-erasing speed. In the case of
using a laser beam, however, it is desirable to dispose a light
absorbing layer having an absorption band in the wavelength of the
laser beam or to allow the recording material to contain a compound
having an absorption band in the wavelength of the laser beam, in
order to enable a highly transparent amorphous recording material
to absorb the laser beam efficiently.
Further, for supplying heat energies in the erasing step, it is
desirable to use as a heat source a hot stamper or a heat roll
which permits instantly heating the entire region of the rewritable
thermal recording medium. For the cooling of the recording medium
once heated, the natural cooling can be employed. Also, it is
desirable to employ rapid cooling by using a cold stamper, a cold
roller, an air cooling using a cold air stream, or a Peltrier
device. Further, an overwriting can be achieved in the rewritable
thermal recording medium of the present invention by using a
plurality of TPH's differing from each other in the energy value or
a plurality of laser beams differing from each other in the
diameter of the beam spot.
Let us describe Examples of the present invention.
Example 1:
1.0 part by weight of CVL (Crystal Violet lactone) as a color
former was mixed with 1.0 part by weight of a phenolic compound, as
a developer, represented by chemical formula (1) given below,
followed by dissolving the mixture in chloroform to obtain a
homogeneous solution. ##STR8##
The solution was cast on a glass substrate 1.5 mm thick to form an
amorphous thin film having a thickness of about 10 .mu.m. The
amorphous thin film was coated with an photocurable epoxy resin,
followed by curing the resin film to obtain a protective film
having a thickness of 1 .mu.m. Then, a heat roll was pressed
against the entire surface of the protective film, followed by
subjecting to natural cooling. As a result, the amorphous thin film
was crystallized so as to be turned white.
FIG. 5 is a cross sectional view showing the construction of the
resultant thermal recording medium. As seen from the drawing, the
thermal recording medium comprises a glass substrate 1, a thin film
recording layer 2 formed on the substrate 1, and a protective film
3 formed on the recording layer 2.
A thermal printing was applied to the thermal recording medium by
using a thermal head (6 dots/mm, 380.OMEGA.) manufactured by K.K.
Toshiba. In this step, a voltage of 12 V with a pulse width of 0.8
millisecond was applied to the thermal head. As a result, the
printed portion was turned amorphous and colored blue so as to
achieve a positive recording. After the recording, the entire
surface of the thermal recording medium was heated by using the
same thermal head. In this step, a voltage of 10 V with a pulse
width of 1.5 milliseconds was applied to the thermal head. As a
result, the printed portion was brought back to the white
crystalline state so as to erase the recorded information. On the
other hand, a hot stamper was pressed for 0.1 second against the
entire surface of another sample of the thermal recording medium
after the recording step, followed by leaving the recording medium
to stand at room temperature. The printed portion was brought back
to the white crystalline state in this case, too, so as to erase
the recorded information. No deterioration was recognized in the
display even after 100 cycles of the recording-erasing operations
described above. Further, no deterioration was recognized in the
printed state even after the thermal recording medium was left to
stand at 30.degree. C. for one year.
Examples 2 to 21:
Various kinds of thermal recording media were prepared exactly as
in Example 1, except that the color formers and the developers
shown in Table 1 were used in these Examples. Each of PSD-V,
PSD-290, PSG-3G and PED-150 used as a color former is a fluorane
series leuco compound manufactured by Nippon Soda K.K. Also, IR
(indolyl red) is a fluorane series leuco compound manufactured by
Yamamoto Kasei K.K. On the other hand, chemical formula numbers of
(2) to (16) shown in Table 1 as developers have chemical structures
as given below: ##STR9##
Recording-erasing was applied to each of the resultant thermal
recording media as in Example 1 by using the thermal head equal to
that used in Example 1. It has been found that a positive recording
can be performed with a pulse width of 0.8 millisecond, and the
erasing can be performed with a pulse width of 1.5 milliseconds.
Table 1 also shows the voltages applied to the thermal head when
information was recorded in and erased from each of the thermal
recording media.
No deterioration was recognized in the display even after 100
cycles of the recording-erasing operations in each of these
Examples 2 to 21, too. Further, no deterioration was recognized in
the printed state even after the thermal recording medium was left
to stand at 30.degree. C. for one year.
TABLE 1 ______________________________________ Developer Thermal
Head Color (Chemical Voltage (V) Examples Former formula) Recording
Erasing ______________________________________ 2 CVL (2) 13 10 3
PSD-V (1) 12 10 4 PSD-290 (1) 12 10 5 CVL (3) 12 9.5 6 CVL (4) 14
11 7 IR (1) 12 10 8 PSD-3G (1) 13 10.5 9 CVL (5) 11 8 10 CVL (6) 13
10 11 CVL (7) 14 10.5 12 CVL (8) 10 7.5 13 CVL (9) 13 9.5 14 CVL
(10) 13 10 15 CVL (11) 14 11 16 PSD-3G (11) 14 11 17 CVL (12) 13
9.5 18 PSD-150 (13) 14 10 19 PSD-150 (14) 14 10 20 CVL (15) 13 9.5
21 CVL (16) 13 9.5 ______________________________________
Comparative Examples 1:
A thermal recording medium was prepared as in Example 1, except
that a phenolic compound represented by the chemical formula (17)
given below was used as a developer: ##STR10##
A thermal printing was applied to the resultant thermal recording
medium by using the thermal head equal to that used in Example 1. A
voltage of 13 V with a pulse width of 0.8 millisecond was applied
to the thermal head for performing the recording operation. The
printed portion was turned amorphous and colored blue so as to
achieve a positive printing. However, when a hot stamper was
pressed for 1 second against the entire surface of the thermal
recording medium, followed by leaving the recording medium to stand
at room temperature, the printed portion was brought back to the
white crystalline state only partially. Erasing has been found to
be incomplete in the case of using a developer which is incapable
of forming a liquid crystal phase.
Example 22:
1.0 part by weight of CVL as a color former was mixed with 2.0
parts by weight of a phenolic compound represented by the chemical
formula (2) given previously, followed by thermally melting the
mixture to obtain a homogeneous composition acting as a recording
material.
On the other hand, a chromium layer acting as a light absorbing
layer was formed in a thickness of 100 nm by a vacuum vapor
deposition on an optically polished glass substrate having a
thickness of 1.2 mm. A small amount of the recording material
composition was melted on the light absorbing layer formed on the
glass substrate, followed by putting another glass plate on the
melt such that the melt was sandwiched in the form of an expanded
thin film between the two glass plates. Then, the glass substrate
was cooled gradually to form a crystalline thin film, about 10
.mu.m thick, acting as a recording layer, thereby preparing a
thermal recording medium comprising a glass substrate, a light
absorbing layer, a recording layer and a glass plate.
Information was written in the recording layer of the resultant
thermal recording medium by irradiating the thermal recording
medium, which was kept rotated at 900 rpm, with a semiconductor
laser light having a wavelength of 830 nm and converged to have a
diameter of 1 .mu.m such that the intensity of the laser light was
6 mW on the surface of the recording medium. When observed with a
polarizing microscope, the written portion, i.e., laser
light-irradiated portion, was found with a high contrast ratio to
have been turned amorphous. Specifically, a linear recording with a
line width of about 1 .mu.m was clearly recognized. After the
recording, the thermal recording medium, which was kept rotated at
900 rpm, was irradiated again with a semiconductor laser light
having a wavelength of 830 nm and converged to have a diameter of 2
.mu.m such that the intensity of the laser light was 3 mW on the
surface of the recording medium. When observed with a polarizing
microscope, the region in which information was written previously,
which was included in the region irradiated with the laser light of
3 mW, was found to have been turned crystalline, indicating that
the recorded information was erased. Incidentally, no change was
recognized in the written state when the recording medium having
the information recorded therein by means of irradiation with 2 mW
of the laser light was left to stand at 30.degree. C. for one
year.
Example 23:
1.0 part by weight of CVL as a color former was mixed with 2.0
parts by weight of a phenolic compound represented by the chemical
formula (2) given previously, followed by thermally melting the
mixture to obtain a homogeneous composition acting as a recording
material. Further, 0.5 g of melamine prepolymer was added to 30 g
of the composition noted above, followed by thermally melting the
resultant composition. Then, the resultant melt was dripped into an
aqueous solution containing 5% by weight of gelatin while stirring
the solution to form fine droplets, followed by further stirring
the solution for 5 hours at about 80.degree. C., with the pH value
of the solution controlled to be 2 with hydrochloric acid. As a
result, formed was a suspension of microcapsules each consisting of
a composition of the color former and the developer and a cured
melamine resin covering the composition. The resultant microcapsule
suspension was coated on a copying paper sheet, followed by drying
and pressing a heat roll against the entire surface of the copying
paper sheet. Then, the copying paper sheet was subjected to natural
cooling at room temperature so as to form a recording layer on the
surface of the paper sheet, said recording layer consisting
essentially of the microcapsules containing a crystalline
composition. Incidentally, the microcapsules separated from the
suspension by filtration, followed by centrifugal separation,
drying, etc. may also be used in the present invention.
A thermal printing was applied to the resultant thermal recording
medium (copying paper sheet) by applying a voltage of 13 V with a
pulse width of 1 millisecond to the thermal head equal to that used
in Example 1. The printed portion was found to have been turned
amorphous and colored blue, indicating that a positive printing was
performed. Then, a hot stamper was pressed for 0.2 second against
the entire surface of the copying paper sheet after the recording,
followed by leaving the copying paper sheet to stand at room
temperature. The printed portion was found to have been brought
back to the crystalline phase so as to erase the recorded
information. Incidentally, no change was recognized in the printed
state even after the copying paper sheet was left to stand at
30.degree. C. for one year.
Example 24:
1.0 part by weight of C.I. basic blue 3 as a color former was mixed
with 4.0 parts by weight of a benzene sulfonic acid derivative
represented by the chemical formula (18) given below: ##STR11##
The resultant mixture was thermally melted to form a homogeneous
molten composition. It should be noted that a decolored state is
formed with increase in the interaction between the color former
and the developer. On the other hand, a color-developed state is
formed with decrease in the interaction between the two.
A small amount of the resultant composition was melted on a glass
substrate 1.5 mm thick. On the other hand, a glass plate, 1 mm
thick, having a small amount of spacers consisting of silica
particles each having a diameter of 10 .mu.m deposited thereon was
disposed on the glass substrate such that the melt in the form of a
thin film was sandwiched between the two glass plates, followed by
subjecting the substrate to natural cooling at room temperature.
When the glass plate was removed, a transparent amorphous thin film
was found to have been formed on the glass substrate in a thickness
of about 10 .mu.m. Then, the amorphous thin film was coated with a
photocurable epoxy resin, followed by curing the resin to form a
protective layer having a thickness of 1 .mu.m. Further, a heat
roll was pressed against the entire surface of the protective
layer, followed by natural cooling at room temperature so as to
crystallize the amorphous thin film and, thus, to form a blue
recording layer.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 13 V with a pulse width of 1
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
a hot stamper was pressed for 0.2 second against the entire surface
of the thermal recording medium after the recording, followed by
leaving the recording medium to stand at room temperature. The
printed portion was found to have been brought back to the blue
crystalline state so as to erase the recorded information.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 25:
A thermal recording medium was prepared as in Example 24, except
that 1.0 part by weight of cyanine dye as a color former was mixed
with 2.0 parts by weight of a phosphorus compound, as a developer,
represented by the chemical formula (19) given below: ##STR12##
It should be noted that a decolored state is formed with increase
in the interaction between the color former and the developer in
this Example, too. Likewise, a color-developed state is formed with
decrease in the interaction between the two.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 13 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
a hot stamper was pressed for 0.2 second against the entire surface
of the thermal recording medium after the recording, followed by
leaving the recording medium to stand at room temperature. The
printed portion was found to have been brought back to the blue
crystalline phase so as to erase the recorded information.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 26:
1.0 part by weight of CVL as a color former, 1.0 part by weight of
a phenolic compound, as a developer, represented by the chemical
formula (2) given previously, and 0.5 part by weight of stearyl
alcohol as a phase separation controller were mixed with each
other, followed by preparing a thermal recording medium as in
Example 1.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 12 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colored blue, indicating that a positive printing was performed.
Then, the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 9 V with a pulse
width of 1.2 milliseconds to the same thermal head. As a result,
the printed portion was brought back to the white crystalline
phase, indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 27:
1.0 part by weight of CVL as a color former was mixed with 1.0 part
by weight of a phenolic compound represented by the chemical
formula (20) given below: ##STR13##
The resultant mixture was dissolved in chloroform to obtain a
homogeneous solution. The developer used was found to be capable of
forming a liquid crystal phase (smectic phase) in a supercooled
liquid of 92 to 110.degree. C. by combination of a differential
scanning calorimetry and a polarizing microscope. The solution was
cast on a glass substrate 1.5 mm thick to form an amorphous thin
film about 10 .mu.m thick. Then, the amorphous thin film was coated
with a photocurable epoxy resin, followed by curing the resin to
form a protective film 1 .mu.m thick. Further, a heat roll was
pressed against the entire surface of the protective film, followed
by natural cooling at room temperature so as to crystallize the
amorphous thin film and, thus, to turn the thin film white.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 12 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colored blue, indicating that a positive printing was performed.
Then, the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 9 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the white crystalline state,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 28:
A thermal recording medium was prepared as in Example 27, except
that a phenolic compound represented by the chemical formula (21)
given below was used as a developer: ##STR14##
The developer used was found to be capable of forming a liquid
crystal phase (nematic phase) in a supercooled liquid of 63.degree.
to 87.degree. C. by combination of a differential scanning
calorimetry and a polarizing microscope.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 11 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colored blue, indicating that a positive printing was performed.
Then, the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 9 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the white crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 29:
A thermal recording medium was prepared as in Example 27, except
that a phenolic compound represented by the chemical formula (22)
given below was used as a developer: ##STR15##
The developer used was found to be capable of forming a liquid
crystal phase (nematic phase) in a supercooled liquid of
165.degree. to 180.degree. C. by combination of a differential
scanning calorimetric analysis and a polarizing microscope.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 15 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colored blue, indicating that a positive printing was performed.
Then, the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 11 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the white crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 30:
1.0 part by weight of CVL as a color former, 1.0 part by weight of
a phenolic compound, used as a developer, represented by the
chemical formula (23) given below, and 10 parts by weight of
.beta.-sitosterol derivative, used as a matrix agent, represented
by the chemical formula (24) were mixed with each other:
##STR16##
The resultant mixture was dissolved in chloroform to prepare a
homogeneous solution. Incidentally, the developer used is not
covered by the general formula (I) given previously. The matrix
agent used was found to be capable of forming a liquid crystal
phase (cholesteric phase) in a supercooled liquid by combination of
a differential scanning calorimetrcy and a polarizing
microscope.
The resultant solution was cast on a glass substrate 1.5 mm thick
to form an amorphous thin film about 10 .mu.m thick. Then, the
amorphous thin film was coated with a photocurable epoxy resin,
followed by curing the resin to form a protective film. Further, a
heat roll was pressed against the entire surface of the protective
film, followed by natural cooling at room temperature so as to
crystallize the amorphous thin film and, thus, to turn the thin
film white.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 12 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 8.5 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the blue crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 31:
A thermal recording medium was prepared as in Example 30, except
that a compound represented by the chemical formula (25) given
below was used as a matrix agent: ##STR17##
The matrix agent used was found to be capable of forming a liquid
crystal phase (smectic phase) in a supercooled liquid by
combination of a differential scanning calorimetry and a polarizing
microscope.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 13 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 9 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the blue crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 32:
1.0 part by weight of CVL as a color former, 1.0 part by weight of
a phenolic compound, as a developer, represented by the chemical
formula (23) given previously, 10 parts by weight of
.beta.-sitosterol derivative, as a matrix agent, represented by the
chemical formula (24) given previously, and 5 parts by weight of
eicosanol as a phase separation controller were mixed with each
other, followed by preparing a thermal recording medium as in
Example 30.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 11 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 8 V with a pulse
width of 0.8 millisecond to the same thermal head. As a result, the
printed portion was brought back to the blue crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 33:
A thermal recording medium was prepared as in Example 32, except
that 10 parts by weight of cholesterol was used as a matrix agent,
and 7 parts by weight of a compound represented by the chemical
formula (26) given below was used as a phase separation controller:
##STR18##
The phase separation controller used was found to be capable of
forming liquid crystal phases (both smectic phase and nematic
phase) by combination of a differential scanning calorimetry and a
polarizing microscope.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 11 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 8.5 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the blue crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 34:
A thermal recording medium was prepared as in Example 32, except
that a compound represented by the chemical formula (27) given
below was used as a phase separation controller: ##STR19##
The phase separation controller used was found to be capable of
forming liquid crystal phases (both smectic phase and nematic
phase) by combination of a differential scanning calorimetry and a
polarizing microscope.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 11 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been turned amorphous and
colorless, indicating that a negative printing was performed. Then,
the entire surface of the thermal recording medium after the
recording was heated by applying a voltage of 8.2 V with a pulse
width of 1 millisecond to the same thermal head. As a result, the
printed portion was brought back to the blue crystalline phase,
indicating that the recorded information was erased.
Recording-erasing was repeated similarly, with the result that no
deterioration of display was recognized after 100 cycles of the
recording-erasing operation. Also, no change was recognized in the
printed state even after the recording medium was left to stand at
30.degree. C. for one year.
Example 35:
1.0 part by weight of phenolphthalein as a color former was mixed
with 1.0 part by weight of an amino compound represented by the
chemical formula (28) given below: ##STR20##
The resultant mixture was dissolved in chloroform to obtain a
homogeneous solution. The solution was cast on a glass substrate
1.5 mm thick to form an amorphous thin film about 10 .mu.m thick.
Then, the amorphous thin film was coated with a photocurable epoxy
resin, followed by curing the resin to form a protective film 1
.mu.m thick. Further, a heat roll was pressed against the entire
surface of the protective film, followed by natural cooling at room
temperature so as to crystallize the amorphous thin film and, thus,
to turn the thin film white.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 12 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been colored red, indicating
that a positive printing was performed. Then, the entire surface of
the thermal recording medium after the recording was heated by
applying a voltage of 10 V with a pulse width of 1.5 milliseconds
to the same thermal head. As a result, the printed portion was
brought back to the white crystalline phase, indicating that the
recorded information was erased. Recording-erasing was repeated
similarly, with the result that no deterioration of display was
recognized after 100 cycles of the recording-erasing operation.
Also, no change was recognized in the printed state even after the
recording medium was left to stand at 30.degree. C. for one
year.
Example 36:
1.0 part by weight of CVL (Crystal Violet lactone) as a color
former, 1.0 part by weight of a phenolic compound, as a developer,
represented by the chemical formula (17) given previously, and 0.3
part by weight of a compound, as a phase separation controller
capable of forming a liquid crystal phase, said compound
represented by the chemical formula (29) given below were mixed
with each other: ##STR21##
The resultant mixture was dissolved in chloroform to obtain a
homogeneous solution. The solution was cast on a glass substrate
1.5 mm thick to form an amorphous thin film about 10 .mu.m thick.
Then, the amorphous thin film was coated with a photocurable epoxy
resin, followed by curing the resin to form a protective film 1
.mu.m thick. Further, a heat roll was pressed against the entire
surface of the protective film, followed by natural cooling at room
temperature so as to crystallize the amorphous thin film and, thus,
to turn the thin film white.
A thermal printing was applied to the resultant thermal recording
medium by applying a voltage of 12 V with a pulse width of 0.8
millisecond to the thermal head equal to that used in Example 1.
The printed portion was found to have been colored blue, indicating
that a positive printing was performed. Then, the entire surface of
the thermal recording medium after the recording was heated by
applying a voltage of 10 V with a pulse width of 1.5 milliseconds
to the same thermal head. As a result, the printed portion was
brought back to the white crystalline phase, indicating that the
recorded information was erased. Recording-erasing was repeated
similarly, with the result that no deterioration of display was
recognized after 100 cycles of the recording-erasing operation.
Also, no change was recognized in the printed state even after the
recording medium was left to stand at 30.degree. C. for one
year.
* * * * *